1. Field of Invention
This invention relates to a non-circular mechanically variable optical attenuator using a pair of specially constructed polarizing elements to achieve a smooth transition from un-crossed to crossed polarized vector fields using a linear motion with respect to one another in place of circular motion. More specifically, the present invention uses mechanical translation and two non liquid polarizing elements through which light can pass with spatially varying polarization axes to achieve variable attenuation.
2. Related Art
In order to obtain a continuously variable optical attenuator, various methods can be used. One such method is to use polarizers. It is generally known in the art that a polarizer refers to a polarizing filter that has two transparent surfaces similar to a glass lens and other optical elements. Electrical and mechanical means for controlling the degree of attenuation through misalignment of the polarization vectors (or crossing of the polarizers) exist.
In common place electric control, a controlling voltage is usually applied to a liquid crystal cell to vary attenuation. This voltage changes the orientation of the liquid crystal molecules (usually aligned in a twisted helical shape) and thereby varies the misalignment (or twist in this case) of linearly polarized light to another, fixed, solid polarizer. This same principle is used in the common Liquid Crystal Displays (LCD). Light passes through a solid linear polarizer, then its polarization gets rotated by the liquid crystal helix, reflected and passes back out through the solid linear polarizer. The controlling electric field temporarily destroys the helix.
One common configuration of a mechanical variable optical attenuator involves a pair of polarizers which are gradually misaligned (or crossed) through rotation of one of the polarizers with respect to the other, leading to increasing optical attenuation. The circular mechanical motion required for crossing polarizers is not always easily achievable due to mechanical constraints. In an application, such as dimming of side and rear view mirrors on a vehicle for glare reduction, rotation is not practical as most of these mirrors are not circular in shape. The use of liquid crystal attenuators in this application is problematic as the liquid crystals are susceptible to damage from the environment such as UV radiation and heat, both present in ordinary sunlight. The availability of ordinary stable polarizers that could be gradually crossed with a linear rather than a circular motion could greatly benefit this and other applications where the previously outlined methods are inconvenient or impractical.
Various variable optical attenuators are disclosed in the literature. For example, U.S. Pat. No. 4,299,444 issued on Nov. 10, 1981 to Römer claims a variable dimmable mirror by electronically controlling a liquid crystal cell. The mirror assembly consists of polarizing filters, a liquid crystal cell, a voltage source for the electrodes of the cell and a reflective surface.
U.S. Pat. No. 6,717,713 issued Apr. 6, 2004 to Hiroshi Onaka et al. Claims a light attenuator by rotation of two linear polarizers with respect to one another but does not employ the use of linear translation of two polarizers with spatial variation of the polarization axis.
U.S. Pat. No. 6,628,452 issued Sep. 30, 2003 to Walter Haeberle et al claims a device usable as an optical switch (for matrix switching) It is not a variable attenuator.
U.S. Pat. No. 6,631,238 issued Oct. 7, 2003 to Hongdu Liu et al. Claims the use of birefringent crystals to separate the input light of the device into first ordinary and first extraordinary beams, separated by a walk off distance. The patent then claims the use of a half wave plate, a variable polarization rotator and a retroreflector to rotate and reflect the beams. However, there is a need for an attenuator which uses only specially constructed polarizers where the polarization axis varies over the surface of the polarizers resulting in attenuation through linear translation of one polarizer with respect to the other.
In a patent issued Mar. 2, 2004 to William L. Tonar, et al., U.S. Pat. No. 6,700,692, a partially reflective, partially transmissive electrochromic mirror, a light sensor, a display, a control circuit, and rearview mirror housing are claimed. The sensors provide input into a control circuit, which adjusts the attenuation of the mirror. The problem with the prior art is that attenuation is achieved through an electric field as opposed to mechanical translation with specially constructed polarizers.
U.S. Pat. No. 6,525,860 issued to Michael Holz et al. on Feb. 25, 2003 claims a device which uses diffraction, an array of liquid crystal regions and an electric field for controlling the attenuation. Given this prior art, there is a need for an attenuator functioning by mechanically translating two solid polarizers with spatially varying polarization axis to achieve variable light attenuation.
U.S. Pat. No. 6,476,966 issued to Florencio E. Hernández et al. on Nov. 5, 2002 claims an optical device for rotation of the polarization of linearly polarized infrared, visible, and ultraviolet radiation, comprising: an input transparent window with oriented surface coating, an output transparent window with oriented surface coating, a twisted Nematic liquid crystal disposed between said coated windows, a spacer located between the input window and the output window, means for facilitating proper alignment and precise adjustment of the spacer, and control means that when continuously rotating in a first direction continuously rotates one of said input window and said output window, in a second direction opposite to the first direction, whereby the direction of polarization of linearly polarized radiation entering conductive yet transparent coatings, which upon application of a voltage causes the rotation to be switched off. This device is then disposed between broadband polarizers to form a wavelength independent optical attenuator, where the incident light is a laser between 1013–1015 Hz. The device further claims a control means including a rotatable screw for allowing an enhanced precision rotation of one of said input window and said output window, a rotatable knob for allowing an enhanced precision rotation of one of said input window and said output window, a motor for rotating the control means in a precision rotation, and a drive belt connecting the motor to the control means. A need remains for an attenuator that achieves variable attenuation through mechanical translation through solid polarizers with a spatially varying polarization axis as opposed to rotation of polarizers and liquid crystal cells as implemented by the prior art.
Additionally, U.S. Pat. No. 6,275,323 issued to Nobuhiro Fukushima on Aug. 14, 2001 claims an optical light amplifier, an optical attenuator variably attenuating the amplified light without mechanically moving along an optical path of the amplified light, wherein the apparatus is a repeater. The optical attenuator comprises a controller controlling the attenuation of the optical attenuator by varying an output of the optical attenuator. The optical attenuator attenuates the amplified light at an output of the optical amplifier. That patent also claims apparatus comprising an optical amplifier amplifying a light and an optical attenuator including a polarizing part and a polarization rotation part, which operate together to variably attenuate the amplified light, wherein the apparatus is a repeater. That patent also claims an optical communication system. However, there remains a need for mechanical crossing of polarizers through translation whereas the prior art relies upon magneto optical crystals and controlling magnetic fields.
U.S. Pat. No. 4,530,600 issued to Marco A. Lopez on Jul. 23, 1985 claims an adjustable rotatable polarization altering means positioned in the first optical path between the light source and a beam splitter means for rotating by controllable amount the polarization of light passing there through, thereby controlling the relative intensities of the component beams of the light intercepted by a beam splitter means and of that light transmitted along said second optical path thereby reducing controllably the intensity of the light directed toward a target. The use of rotational polarization by the prior art leaves a void that could be fulfilled by the use of varying spatial polarization through solid polarizers.
Further, U.S. Pat. No. 5,642,236 issued Jun. 24, 2997 to Niel Mazurek claims a dimmable safety mirror being adaptable for mounting to a conventional motor vehicle mirror comprising: a dimmable mirror subassembly having a dimmable surface selectively switchable between at least a high reflectance state and a low reflectance state, and a means for mounting the dimmable mirror to the conventional mirror in a manner to make the dimmable surface a viewing surface of the safety mirror wherein the dimmable mirror subassembly can be quickly and easily removed to make the conventional mirror the viewing surface. This patent also claims the dimmable mirror surface of the mirror subassembly is a dichroic liquid crystal mirror, and the dimmable safety mirror receives electrical power from the motor vehicle's electrical system. Given the prior art's reliance on dichroic and/or liquid crystal material and electrical power to achieve attenuation, there remains a need for a simpler attenuator which utilizes varying spatial polarization.
Similarly, U.S. Pat. No. 5,521,744 issued on May 28, 1996 also to Neil Mazurek focuses on the housing of the safety mirror. The patent claims a dimmable safety mirror comprising a mirror comprising a dimmable front reflective surface and rear reflective surface and the front surface is switchable between a high reflectance state and a low reflectance state. That patent also claims the dimmable mirror surface is a dichroic liquid crystal mirror and the dimmable safety mirror is powered. Again, a need is created for an attenuator that does not achieve attenuation through varying surface reflectivity as in the prior art.
U.S. Pat. No. 4,572,619 issued to Siegfried Reininger et al. on Feb. 25, 1986 claims an electrically dimmable rearview mirror. However, the use of an electro-optical effect of a thin ceramic layer, which is controlled through an applied electric field and claimed by the prior art is not useful for the applications that require a polarizing attenuator of mechanical means.
U.S. Pat. No. 6,512,624 issued to Toner et al. on Jan. 28, 2003 claims an electrochromic rearview mirror comprising: front and rear elements each having front and rear surfaces and being sealably bonded together in a spaced-apart relationship to define a chamber, a transparent first electrode including a layer of conductive material carried on a surface of one of said elements, an electrochromic material contained in said chamber, and a partially transmissive, partially reflective second electrode disposed over substantially all of said front surface of said rear element, said second electrode including a transparent coating applied over a surface of said rear element, element and a thin reflective layer of metal applied over said transparent coating, wherein said electrochromic rearview mirror exhibits a reflectance of at least about 35 percent, a C* value of less than about 20. This patent also claims a silver alloy as the reflective surface on the order of 180–500 Angstroms thick. The reliance of the prior art on electrochromic material or an applied electric field to achieve attenuation creates a need for an attenuator that does not utilize electrochromic material or an applied electric field.